This invention relates to methods and apparatus for removing haze from hydrocarbon oil mixture boiling in the lubricating oil range. Throughout this specification, it is to be understood that "hydrocarbon oil mixture" and "oil mixture" both means a hydrocarbon oil mixture boiling in the lubricating oil range.
The problems of wax in lubricating oil are very well known in the art. In the distillation of crude oil, a proportion of wax is present in cuts taken in the lubrication oil range. Some of the wax remains dissolved in the oil, whereas other fractions form a haze as the oil fraction ages at ambient temperatures. Wax in itself is in fact a good lubricant but under comparatively low temperatures such as engine cold start conditions, its presence causes the oil to be thick and viscous and as a result the engine may be hard to turn over at sufficient speed during starting. In addition, wax haze or particulate matter can lead to plugging of the engine oil filter. Haze manifests itself as a milky or cloudy appearance in the oil and is often caused by wax or by both wax and tiny water droplets being present in the lubricating oil. Typically a minimum of about 0.1% by volume of wax will cause some lubricating oil to look hazy. Therefore the existence of haze caused by the presence of wax crystals or particles detrimentally affects the performance of lubricating oils.
Techniques have been available for many years which enable dissolved wax to be separated from lubricating oil. A well-known approaah is to mix an oil solvent with the lubricating oil, chill the mixture to precipitate wax crystals, and separate the wax from the resulting slurry by settling or filtration. "Oil solvent" as used throughout this specification refers to those solvents which when added to an oil mixture result in a lower viscosity for the solvent-oil mixture than for the oil mixture alone. Viscosity reduction is beneficial for enhancing the settling or filtration processes used to separate the precipitated wax from the solvent/oil. Usually, the oil solvent will have the additional property of having a higher solubility for the oil (hydrocarbon oil mixture) than for the wax at any given temperature, so that during chilling of the solvent-oil mixture to precipitate wax, the wax precipitation is enhanced. Liquified hydrocarbon alkanes or alkenes, ketones, toluene or other aliphatics, and light organic chlorides are examples of well known oil solvents. When the wax-laden oil is cooled, then as the temperature of the oil is reudced, different wax fractions start to come out of solution, aided by the oil solvent. Propane is an example of a preferred oil solvent because in addition to its oil solvent properties, it boils or vaporizes at typically encountered conditions of temperature and pressure in propane dewaxing processes, and this in turn leads to an auto-refrigeration effect which can be used to bring about the desired cooling, at least in part, of the wax-laden oil.
The grown wax particles and crystals are separated from the lubricating oil/oil solvent mixture, following the above described propane pretreatment, by means of a suitable mechanical filter. Rotary drum filters are well-known in the art for this purpose. The lubricating oil/oil solvent mixture is brought into contact with a filter cloth, extending around the rotary filter drum, in one angular position of the filter drum and a wax cake is formed on the filter cloth. In another angular position, the wax cake is washed with a solvent to recover oil from the wax cake and in a further angular position of the rotary filter drum, wax is scraped from the filter cloth by a scraper or "doctor" blade positioned adjacent the periphery of the rotary filter. The thereby-resulting relatively wax-free cloth then rotates further into contact with the wax-laden mixture of oil and solvent again to collect more wax and this process continues. Since the rate of filtration is directly related to the viscosity of the lube oil/oil solvent mixture, which is lower than that of the lube oil alone, the filtration rate is enhanced.
The wax particles need to be grown as large as possible in the oil solvent pretreatment process, in order to achieve the most effective wax separation by the rotary filters. This requires that the temperature of the wax-laden lubricating oil be reduced very gradually (typically around 2.degree. C. per min.). If the temperature were to be reduced more rapidly, much smaller wax crystals would be produced for a given temperature drop. Therefore, the oil solvent pretreatment apparatus has to be specifically designed to provide the necessary stringent operating parameters. In addition, dewaxing aids are often used to promote the wax crystallation.
After removal of wax in the rotary drum filter the dewaxed lube oil (DWO) and oil solvent admixture are in general processed in a solvent recovery plant which recovers the oil solvent from the admixture, leaving DWO which is accumulated in oil storage tanks.
The approach to lube oil dewaxing described above is well known and widely used in the effective and economical dewaxing of lube oil. However, recent developments in the electrical separation field have led to new dewaxing techniques which appear to be promising. For example, U.S. patent application Ser. No. 722163 filed on the Apr. 17, 1985 in the name Ryan et al and assigned to the present assignees relates to separating wax particles from hazy lube oil by introducing free excess charge into the oil (for example by injecting charge into the oil by field emission from a sharply pointed, high potential, electrode) and bringing the charged oil into contact with at least one collector surface on which wax particles agglomerate. For collecting the wax in this way, the charged oil may be passed into a separation vessel containing a bed of densely packed beads of low electrical conductivity so that the wax agglomeration takes place in the bed on the bead surfaces and wax deposits build up on the beads. After a while, the wax deposits are sufficiently large that the separation efficiency diminishes so that it becomes necessary to replace the beads with fresh ones, remove the existing beads to clean them before returning them, or regenerate the beads in situ. Lube oil dewaxing is important not only for separating wax from the oil but also because the wax in itself is a useful product which can be sold or treated further, for example in a catalytic cracker, to produce further petroleum products. Therefore, it is wasteful to dispose of the wax-soiled beads because of the intrinsic value of the wax and in any case the cost of the beads themselves is not insignificant. On the other hand cleaning the beads is costly and when using a solvent wash to do this as is known in the art, the solvent removes the wax from the beads but the solvent has to be removed from the wax. If the beads are regenerated in situ, then there is the added disadvantage of downtime of the dewaxing apparatus. The present invention employs the same electrical separation method and apparatus as the aforesaid Ryan et al application but employs novel means to achieve an improvement in the respects discussed above.